(1) Field of the Invention
The present invention relates to a refractory element used for the continuous casting of molten steel from an upstream metallurgical vessel to a downstream metallurgical vessel.
According to a particular embodiment of the invention, the nozzle is used for casting molten steel from a distribution tank (sometimes also called a tundish) to a casting mould or ingot mould (sometimes also called a coquille).
(2) Description of Related Art
In the continuous casting of steel from the tundish to an ingot mould, a pouring nozzle is used to protect the liquid steel from chemical attacks from the surrounding atmosphere and to isolate it thermally during its transfer from the upstream vessel to the downstream vessel. These nozzles, roughly cylindrical in shape, consist of a single piece having an upstream end possessing a generally tapered inlet disposed in the vicinity of the bottom of the upstream vessel. These nozzles are pierced right through by a bore forming a pouring channel enabling liquid steel to flow towards the downstream end of the nozzle, which is immersed in the ingot mould. In the majority of cases, the bottom end of the nozzle is closed or at the very least is provided with a restriction in order to limit the vertical flow of the jet of steel and the steel emerges in the ingot mould mainly through lateral openings (also called outlets) with which the downstream end of the nozzle is provided. In the context of the present invention, the term a “closed” bottom end of a nozzle will be used in order to designate either nozzles that are actually closed at their bottom end or simply provided with such a restriction. In the case of the casting of steel as flat products, such as slabs, an ingot mould is used, which is a bottomless mould having four lateral walls, generally made from copper, water cooled, parallel in pairs, and which has a roughly rectangular-shaped cross section corresponding approximately to the width and thickness of the slab. The ingot mould has a length substantially greater than its width. The lateral openings in the bottom part of the nozzle are normally disposed symmetrically with respect to one another to allow homogeneous flow in the ingot mould. In addition, the lateral openings never exactly emerge facing the long walls of the ingot mould, which are also closest to the nozzle, without which the liquid steel discharged from the tundish and therefore still at high temperature would directly come into contact with the long walls, and would cause excessive heating and, after a certain amount of time, the melting of the copper walls. The result would be a leakage of steel with disastrous consequences both for the plant and for the personnel. On the contrary, the lateral openings of the openings of the nozzle are oriented towards the narrow walls of the ingot mould, which are also the furthest away; thus the steel discharged from the tundish has time to cool in contact with the previously poured steel before reaching the walls.
Such pouring nozzles are wearing parts highly stressed to the point that their service life may limit the pouring time. In particular, these nozzles may be clogged by deposits of alumina, eroded chemically by particularly aggressive slag or grade of steel, or cracked by a thermal or mechanical shock. Thus, since the 1980s, devices for supplying and exchanging nozzles have been developed.
In these devices, the submerged entry nozzle, up until then consisting of a single piece and extending from the bottom wall of the tundish as far as the heart of the ingot mould, is replaced by an assembly comprising an inner nozzle (corresponding to the top portion of the submerged entry nozzle) conveying the steel through the bottom wall of the tundish, and a pouring nozzle (corresponding to the bottom portion of the submerged entry nozzle) for transferring steel into the ingot mould. In general, the inner nozzle and the pouring nozzle consist of a single piece, but they may also result from an assembly for example of a plate and tube. The plate may also be cast around a prefabricated tube. In the pouring position, the pouring channels of the inner nozzle and of the pouring nozzle fluidly communicate. The downstream end of the inner nozzle consists of a plate provided with an orifice and which can be applied sealingly against another plate also provided with an orifice constituting the upstream end of the pouring nozzle. The two plates ensure firstly the tightness of the connection between the two nozzles and secondly the sliding of the pouring nozzle from a standby position to a pouring position. These plates are generally rectangular in shape so as to be able to slide in the guide system. In the context of the present description, reference will be made to this general rectangular shape even if in practice the plate deviates from this shape, for example if it has rounded or truncated corners. In all cases, the plate will be circumscribed by a rectangle that has four sides intersecting each other at right angles and the opposite sides of which are parallel in pairs. By the way, it should be noted that the pouring nozzle slides in the guide systems in a direction parallel to a pair of sides that also corresponds to the direction given by an axis passing at the centre of gravity of the lateral openings (the axis of the outlets). It will also be noted that, in some cases, the lateral openings of the nozzle are offset intentionally so that they are not exactly oriented towards the narrow walls of the ingot mould. For example, the axis of the outlets can be offset by up to 25° in order to promote the circulation of steel in the ingot mould in order to improve the homogeneity of the cast product. The device for supplying and exchanging nozzles can also be offset in order to avoid interference at this device. In this case, if it is wished to keep the axis of the outlets strictly parallel to the axis of the ingot mould, it will be necessary to offset this axis with respect to the direction of sliding in the guide system. In the context of the present invention, when a direction is defined with respect to the axis of the outlets, it will be kept in mind that this direction may vary from −25° to +25°. Thus, when it is said of a direction that it is parallel to the axis of the outlets, it will be necessary to understand that this direction is parallel to, within 25°, the axis of the outlets.
In a plant using such devices for supplying and exchanging nozzles, the pouring is carried out through the inner nozzle and a first pouring nozzle, the bores of which communicate. When the pouring nozzle in the pouring position must be replaced, the device slides a new pouring nozzle, up till then in the standby position, on a system of guides comprising guide rails towards the pouring position. During this sliding, the new pouring nozzle drives away the pouring nozzle to be replaced. During the sliding, the plate forming the upstream end of the pouring nozzle comes in line with the pouring channel of the inner nozzle and closes it off. European patent EP-B1-192019 represents such a device. This device has perfectly met the requirements of the market and has afforded a significant extension in the lengths of the casting sequences.
In the majority of cases, the regulation of the flow of poured steel and in particular the interruption of the pouring at the end of the pouring sequence is achieved by means of a stopper rod actuated from the top of the tundish, the body of which passes through the liquid steel bath and the nose of which is adapted to close off the inlet of the inner nozzle.
It sometimes happens that the casting operators are confronted with emergency situations in which it is necessary to interrupt the pouring without the slightest delay. For example, in the case of breakage of the stopper rod or any incident during the casting operations. The prior art recommends in this case the use of a blind plate taking the place of the new nozzle. When the blind plate arrives in the pouring position (which should rather be called the closure position), the downstream orifice of the inner nozzle is thus obstructed by said plate and the pouring sequence is interrupted. To deal with an emergency situation, the pouring operators generally leave this blind plate permanently in the standby position on the guide system in order to be able to slide it into the closure position immediately if needed. When the pouring nozzle must be replaced, it is then necessary to remove the blind plate and to replace it with a new nozzle. An emergency situation arising precisely at this moment generally results in a major incident since, before being able to interrupt the pouring by means of the blind plate, it is necessary to release the new nozzle from the guide system, to move it away from the pouring plant, to recover the blind plate, to arrange the latter on the guide system and to slide it into the closure position. Precious seconds are thus lost and may make it impossible to interrupt the sequence, the device having been damaged in the meantime or no longer being accessible to the operators.
The prior art (U.S. Pat. No. A1-5,494,201) has proposed a solution to this problem, consisting of providing the device with a system of additional guides (for example disposed perpendicular to the first guide rails, enabling the blind plate to be introduced at any time since, even at the precise moment of a replacement of a pouring nozzle, the blind plate is still in the standby position and ready to be slid into the closure position. Such a device is however relatively bulky and is therefore not suitable for all pouring plants.
It has also been suggested to use a pouring nozzle the plate of which constituting the upstream end has been extended in the opposite direction to the sliding direction, by a distance at least equal to the pouring hole. In this way, it is possible to close off the pouring channel by slightly pushing the pouring nozzle, a portion of said upstream plate of the pouring nozzle not having an orifice then coming in line with the orifice of the pouring channel provided in the bottom end of the inner nozzle. This development has not encountered significant commercial success since it requires extending the upstream plate of the pouring nozzle and consequently the stroke of the jack. It is consequently not applicable to plants where the space available under the tundish or in the ingot mould is restricted.
The emergency closure system normally used at the present time is therefore the blind plate with all the drawbacks dealt with above.
The industry is therefore still searching for an emergency closure system for a device for supplying and exchanging continuous casting nozzles that can be used in any plant and in particular in plant where the available space is limited. In addition, it would be necessary for this emergency closure system to be able to be used very rapidly at any time, in particular even at the time when the operator envisages replacing the pouring nozzle.